n*(n-1)*(n-2)*(n-3)*(n-4)*(2*n-1)/72.

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =0a(5) =15a(6) =110a(7) =455a(8) =1400a(9) =3570a(10) =7980a(11) =16170a(12) =30360a(13) =53625a(14) =90090a(15) =145145a(16) =225680a(17) =340340a(18) =499800a(19) =717060a(20) =1007760a(21) =1390515a(22) =1887270a(23) =2523675a(24) =3329480a(25) =4338950a(26) =5591300a(27) =7131150a(28) =9009000a(29) =11281725